Diigo
www.diigo.com
Diigo is my personal favorite for social bookmarking. Users can not only import del.icio.us or other bookmarks, they can also update those other services using diigo. In addition, diigo’s informational video and screencasts—accessible on its home page—provide a great introduction to social bookmarking. Diigo’s secondary features include a rich set of browser tools that allows users to highlight passages and leave comments on Web pages for other diigo users to see (a great way for teachers to effectively assess student assignments). Diigo also lets you send an email or blog post directly from a Web page, automate a daily blog post of your bookmarks with comments, or create blog or site widgets with your bookmarks. Founded in 2005 by Wade Ren.